Earthquake Creep Is Shallower Than Thought

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Along the San Andreas Fault between San Juan Bautista and
Parkfield in central California, scientists issue no dire
warnings of future bridge-collapsing earthquakes.
This section of the 800-mile-long (1,300 kilometers) fault
produces no strong earthquakes at all.

Instead of sticking and locking together and breaking in
occasional big temblors, the fault creeps, steadily releasing
strain through thousands of tiny microquakes. One of the big
puzzles in geology is understanding why faults like the San
Andreas creep, and how the process links to large earthquakes
elsewhere on the fault.

A new computer model finds that creep starts shallowly — about
3,200 feet (1 km) below Earth's surface — on strike-slip faults
such as the
San Andreas Fault. Earlier models — based, in part, on rock
lab studies — had suggested the creeping zone was deeper, between
1.8 to 3 miles deep (3 to 5 km).

Instead of using rocks in the lab, the new model relied on
real-world earthquake and
fault-creep data collected after the 1987 Superstition Hills
earthquake in Southern California. The Superstition Hills Fault
is a strike-slip
fault in the Imperial Valley, near El Centro. The results of
the new model were published June 2 in the journal Nature
Geoscience.

The model requires a new, "conditionally unstable" zone in the
sediments at the top of the fault. No regular earthquakes can
strike in this zone, but the fault can slowly crawl. In the San
Andreas Fault's San Juan Bautista creeping section, this zone is
about 985 feet (300 m) thick, according to the model. On the
Superstition Hills Fault, the zone is about 3,200 feet (1 km)
thick. But both creeping zones are at the same depth, around
3,200 feet (1 km) below the surface.

The researchers said the results are an important step in
ground-truthing mechanical fault models."Creep is a basic feature
of how faults work that we now understand better," Jeff McGuire,
study co-author and a researcher at the Woods Hole Oceanographic
Institution in Massachusetts, said in a statement.